Oil heir Andrew Getty had been diagnosed with a brain aneurysm and had not been eating or sleeping well in the days before his death, the mother of his girlfriend said.
Marilynne DeJonge told The Associated Press she has not spoken to her daughter Lanessa DeJonge since Saturday and is worried about her.
She said Lanessa DeJonge met Mr Getty seven years ago and depended on him for housing, and she does not have a phone.
The mother says her 32-year-old daughter was charmed by Mr Getty after he saw her modelling photos and wanted to meet her. She took care of him, trying to get him to eat healthy food, but he preferred sweets.
She said her daughter had a downstairs room in Mr Getty's spacious Hollywood Hills mansion in Los Angeles.
Lanessa DeJonge found Mr Getty, 47, dead on Tuesday and called 911.
Mr Getty appeared to have died of natural causes, coroner's Assistant Chief Ed Winter said, but he added it could take more than two months to determine the exact cause.
He said Mr Getty had not been feeling well for the last couple months and had a doctor's appointment for the day after he died.
The reclusive Mr Getty had said in recent court documents that he was fighting a "serious medical condition" that could endanger his life.
A law enforcement official briefed on the investigation into the death said he had recently complained of stomach pains.
Another official with knowledge of the investigation added that authorities found prescription medications at his home.
Mr Getty obtained a restraining order last month against Lanessa DeJonge, noting her behaviour and arguments between them, and expressing concern about his health.
She had recently moved back into her room at the Getty house, an official said.
She came and went as she pleased despite the active restraining order, which the official said was not unusual in some relationships.